Heretofore, outboard engine units have been known which include an exhaust means (exhaust duct) provided in an engine cover that covers an upper section of an engine, and in which an inlet port of the exhaust duct opens to an engine room and an outlet port of the exhaust duct opens into an exhaust chamber.
One example of such outboard engine units is disclosed in Japanese Patent No. 3,608,637 (hereinafter referred to as “the relevant patent literature”). In the outboard engine unit disclosed in the relevant patent literature, air heated in the engine flows from the engine room into the exhaust duct via the duct's inlet port and then into the exhaust chamber via the duct's outlet port. With the outboard engine unit attached to a hull or body of a watercraft, an exhaust outlet port of the exhaust chamber is located rearwardly of the watercraft body. Thus, when the watercraft body is slidingly travel along the water surface, a negative pressure is produced in an area outside the exhaust outlet port, so that the air having flown into the exhaust chamber is discharged via the exhaust outlet port. By discharging the air within the engine room (i.e., from within the engine room) via the exhaust outlet port as noted above, the engine room can be cooled, and thus, the engine can constantly achieve appropriate performance.
Namely, the exhaust duct disclosed in the relevant patent literature is constructed to discharge, via the exhaust outlet port, the air from within the engine room (engine cover) utilizing the negative pressure produced outside the exhaust outlet port during sliding travel of the watercraft. In order to keep the engine performance even more appropriate, there is a need to discharge the air from within the engine cover in a more stable manner. As a means for meeting the need, there has been known an exhaust means where an exhaust fan is provided at a halfway or en route position in the exhaust duct. By rotation of the exhaust fan provided at en route position in the exhaust duct, the air within the engine cover can be compulsorily discharged through the exhaust outlet port. However, in order to discharge the air from within the engine cover more efficiently, there is a further need to improve an outer peripheral shape of the exhaust fan and a shape of an exhaust passageway.